1. Field of the Invention
The invention relates to a power supply, and more particularly, to a power supply which can convert an oscillating voltage into a variable steady state voltage.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art power supply 10. The power supply 10 is used for converting an oscillating voltage 12 into a steady state voltage which is then outputted to a display device 16 through an output port 14. The power supply 10 comprises a voltage converting circuit 18, a transformer 22, a pulse generator 26, a light coupling device 36, a rectifying circuit 30 and a feedback circuit 34.
The voltage converting circuit 18 comprises a bridge circuit 38 and a filter 40 for rectifying the oscillating voltage 12 into a first steady state voltage. The transformer 22 has an input end 20, an output end 24 and a control end 28. The input end 20 is electrically connected to an output end 42 of the voltage converting circuit 18. The transformer 22 is used for transforming the first steady state voltage at the input end 20 into a second oscillating voltage at the output end 24. The control end 28 is used for controlling on and off of the transformer 22. The pulse generator 26 has an output end 44 and a control end 46. The output end 44 is electrically connected to the control end 28 of the transformer 22. The pulse generator 26 is used for generating a pulse signal at the output end 44 so as to control the on and off of the transformer 22. The control end 46 is used for controlling a duty cycle of the pulse signal. The rectifying circuit 30 has an input end 32 electrically connected to the output end 24 of the transformer 22 for rectifying the second oscillating voltage so as to generate the steady state voltage at the output port 14 of the power supply 10.
The feedback circuit 34 has an input end 50, a control end 56 and an output end 52. The input end 50 is electrically connected to the output end 48 of the rectifying circuit 30. The light coupling device 36 is electrically connected between the output end 52 of the feedback circuit 34 and the control end 46 of the pulse generator 26 for isolating the current between the output end 52 of the feedback circuit 34 and the control end 46 of the pulse generator 26. The feedback circuit 34 comprises a comparator 54 for comparing the voltage at the control end 56 with a predetermined voltage so as to generate a control signal, and a variable resistor 58 for fixing errors caused by other resistive components.
When the voltage at the control end 56 is higher than the predetermined voltage, the impedance of the comparator 54 will drop thus grounding the output end 52, turning on the light coupling device 36, and generating an offset voltage at the control end 46 of the pulse generator 26. When detecting the offset voltage, the pulse generator 26 will gradually reduce the duty cycle of the pulse signal generated at the output end 44. When the voltage at the control end 56 is lower than the predetermined voltage, the impedance of the comparator 54 will increase thus turning off the light coupling device 36 and grounding the control end 46 of the pulse generator 26. And the pulse generator 26 will gradually increase the duty cycle of the pulse signal generated at its output end 44.
The feedback circuit 34 can control the pulse generator 26 to sustain the steady state voltage at the output port 14 at only one fixed voltage, and the power supply 10 has no component to change the steady state voltage. Therefore, the power supply 10 can not generate different steady state voltages for different display devices.